The present invention relates, in general, to improved single longitudinal mode laser diodes and to methods for producing the same.
Single mode lasers with a high side-mode suppression ratio (SMSR) are key devices for many applications, including, for example, spectroscopy, sensing and optical communications. To date, so-called distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, and vertical cavity surface-emitting lasers (VCSEL) have been exclusively used for such applications. However, the fabrication of laterally coupled DBR lasers and the fabrication of buried hetero-structure DFB lasers require the use of sophisticated fabrication techniques. The sub-micron e-beam or focused ion beam lithography processes used for DBR lasers are not desirable for mass production because of their serial nature, and the delicate crystal re-growth processes needed to fabricate the corrugated surface in DFB lasers make them unsuitable. These sophisticated processes inevitably give low yield and high cost for the production of these lasers, so there has been a strong demand for more cost-effective single mode lasers, especially for use in the growing fields of data-communications. In addition, VCSELs suffer from their inability to provide high power because of their short cavities.
Presently available devices also require cooling for proper operation, and the need to provide heat dissipation apparatus increases their total cost. Therefore, there is a strong desire for SMSR laser devices that do not require cooling. It is well known that laser diodes made of AlGaInAs/InP material present better high temperature performance compared to conventional InGaAsP/InP material in the fiber communication wavelengths around 1550 nm and 1310 nm. However, the Al-containing material makes the previously used regrowth fabrication process extremely difficult and unreliable because of Al oxidation. Accordingly, a new structure or a new fabrication method that is compatible with AlGaInAs/InP is strongly desired for the development of single mode laser diodes that do not require cooling.
In the 1980s extensive efforts were devoted to coupled-cavity lasers to get single mode operation; for example, see Larry Coldren et al. IEEE Journal of Quantum Electronics, QE-20, p-659 (1984). However, mode discrimination was not strong enough in the coupled cavity scheme and it was difficult to get high SMSR under modulation. Also, it was impossible to keep a high SMSR over the wide temperature range (0-85 deg. C.) that is expected for uncooled operation.